Takumi Hawa

On the role of built-in electric fields on the ignition of oxide coated nanoaluminum: Ion mobility versus Fickian diffusion

Using the classical molecular dynamics method we simulate the mechanochemical behavior of small (i.e., core diameter<10 nm) oxide coated aluminum nanoparticles. Aluminum nanoparticles with core diameters of approximately 5 and 8 nm are simulated with 1 and 2 nm thick oxide coatings or shells. In addition to thickness the shells are parametrized by varying degrees of crystallinity, density, and atomic ratios in order to study their effect on the ignition of nanoparticle oxidation. The oxide shells are parametrized to consider oxide coatings with the defects that commonly occur during the formation of an oxide layer and for comparison with a defect free crystalline oxide shell. Computed results include the diffusion coefficients of aluminum cations for each shell configuration and over a range of temperatures. The observed results are discussed and compared with the ignition mechanisms reported in the literature. From this effort we have found that the oxidation ignition mechanism for nanometer sized oxide ...

Internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles

We present a study of internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles of 2-10 nm diameter as a function of temperature, using molecular dynamics simulations employing a reparametrized Kohen-Tully-Stillinger interatomic potential. The internal pressure was found to increase with decreasing particle size but the density was found to be independent of the particle size. We showed that for covalent bond structures, changes in surface curvature and the associated surface forces were not sufficient to significantly change bond lengths and angles. Thus, the surface tension was also found to be independent of the particle size. Surface tension was found to decrease with increasing particle temperature while the internal pressure did not vary with temperature. The presence of hydrogen on the surface of a particle significantly reduces surface tension (e.g., drops from 0.83 J/m(2) to 0.42 J/m(2) at 1500 K). The computed pressure of bare and coated particles was found to follow the classical Laplace-Young equation.

Molecular dynamics simulation of the energetic reaction between Ni and Al nanoparticles

Molecular dynamics simulations are used to simulate the energetic reaction of Ni and Al particles at the nanometer scale. The effect of particle size on reaction time and temperature for separate nanoparticles has been considered as a model system for a powder metallurgy system. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles embedded within a matrix. The differences in melting temperature and phase change behavior, e.g., the volumetric expansion of Al between Al and Ni, are expected to produce differing results for the coated nanoparticle systems. For instance, the volumetric expansion of Al upon melting is expected to produce large tensile stresses and possibly rupture in the Ni shell for Ni-coated Al. Simulation results show that the sintering time for separate and coated nanoparticles is nearly linearly dependent on the number of atoms or volume of the sintering nanoparticles. We have also found that nanoparti...

Molecular dynamics simulation of the kinetic sintering of Ni and Al nanoparticles

The kinetic sintering of Ni and Al nanoparticles is considered using molecular dynamics simulations. We report on the effects of nanoparticle size on sintering temperature and time, with results showing that surface energy has a slight effect on both results. The effect of surface energy on combustion temperature is limited to nanoparticles of less than 10 nm in diameter. An analysis of the various alloys formed during sintering gives insight into the reaction process. The formation of Al-rich compounds is observed initially with a final equilibration and rapid formation of the eutectic alloy immediately preceded by melting of the Ni nanoparticle. We have observed that nanoparticle size and surface energy are both important factors in determining the adiabatic reaction temperature for this material system at nanoparticle sizes of less than 10 nm in diameter.

Mechano-chemical stability of gold nanoparticles coated with alkanethiolate SAMs.

Molecular dynamics simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. We observed that the surface of gold nanoparticles becomes highly corrugated by the adsorption of the SAMs. Furthermore, as the temperature is increased, the SAMs dissolve into the gold nanoparticle, creating a liquid mixture at temperatures much lower than the melting temperature of the gold nanoparticle. By analyzing the mechanical and chemical properties of gold nanoparticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we determined that the system is metastable. The model and computational results that provide support for this hypothesis are presented.

Ductility at the nanoscale: Deformation and fracture of adhesive contacts using atomic force microscopy

Fracture of nanosize contacts formed between spherical probes and flat surfaces is studied using an atomic force microscope in an ultrahigh vacuum environment. Analysis of the observed deformation during the fracture process indicates significant material extensions for both gold and silica contacts. The separation process begins with an elastic deformation followed by plastic flow of material with atomic rearrangements close to the separation. Classical molecular dynamics studies show similarity between gold and silicon, materials that exhibit entirely different fracture behavior at macroscopic scale. This direct experimental evidence suggests that fracture at nanoscale occurs through a ductile process.

Spatial and size-resolved electrostatic-directed deposition of nanoparticles on a field-generating substrate: theoretical and experimental analysis

We build on our prior work on electrostatically directed nanoparticle assembly on a field-generating substrate (Tsai et al 2005 Nanotechnology 16 1856‐62). In this paper we develop a data set for particle size-resolved deposition, from which a Brownian dynamics model for the process can be evaluated. We have developed a trajectory model applied to positioning metal nanoparticles from the gas phase onto electrostatic patterns generated by biasing p‐n junction substrates. Brownian motion and fluid convection of nanoparticles, as well as the interactions between the charged nanoparticles and the patterned substrate, including electrostatic force, image force and van der Waals force, are accounted for in the simulation. Using both experiment and simulation we have investigated the effects of the particle size, electric field intensity, and the convective flow on coverage selectivity. Coverage selectivity is most sensitive to electric field, which is controlled by the applied reverse bias voltage across the p‐n junction. A non-dimensional analysis of the competition between the electrostatic and diffusion force is found to provide a means to collapse a wide range of process operating conditions and is an effective indicator of process performance. (Some figures in this article are in colour only in the electronic version)

Evaluation of Viscosity and Rutting Properties of Nanoclay-Modified Asphalt Binders

In pavement constructions, asphalt binders are often modified with synthetic polymers to sustain excessive heat during hot summer days. However, the cost of a polymer-modified binder (PMB) is significantly higher than a neat binder. Nanoclays, however, are relatively inexpensive and naturally abundant and have favorable intrinsic properties (e.g., nanoscopic size and surface area) toward increasing its stiffness. The current study evaluated viscoelastic properties of a performance grade (PG 64-22OK) binder modified with different dosages of Cloisite 15A nanoclay. The state of dispersion and exfoliation of the nanoclay binders were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques. Preliminary results show that 1% and 2% of Cloisite 15A increased the Superpave rutting factor of the base binder by 16% and 26%, respectively. The rotational viscosity (RV) tests reveal a significant increase in the viscosity of the base binder. The stiffness of the 2% nanoclay-modified binder was found to be about the same as that of a PMB-modified (PG 70-28OK) binder, indicating nanoclays can potentially be alternatives to PMBs toward reducing pavement construction and maintenance costs.

Evaluation of Moisture Susceptibility and Healing Properties of Nanoclay-Modified Asphalt Binders

Due to an increasing rate of traffic volume and truckloads in recent years, asphalt binders are often modified with polymers to increase stiffness and sustain excessive heat during hot summer days. However, the cost of polymer-modified binders (PMB) is significantly higher than un-modified binders. Nanoclays, on the other hand, are relatively inexpensive, naturally abundant, and environmentally sustainable, and have favorable intrinsic properties such as nanoscopic size and high surface area. The current study evaluated moisture resistance and healing potential of a commonly used performance grade (PG 64-22) binder modified with different dosages of two selected nanoclay (Cloisite® 15A and Cloisite® 11B) through a surface free energy (SFE) technique. The adhesive energies, an indicator of bond strength, values of nanoclay-modified binders and two selected aggregates (sandstone and limestone) were also evaluated. It is observed that the work of cohesion increases when the asphalt binder is modified with nanoclays, which implies increased healing potential of microcracks. A 4% Cloisite® 15A seems to be effective in maintaining good wetting ability with different aggregates. Cloisite® 15A, consisting of smaller particle size than Cloisite® 11B, is found to be more compatible than Cloisite® 11B.

Investigation of the effect of bilayer membrane structures and fluctuation amplitudes on SANS/SAXS profile for short membrane wavelength

The effect of bilayer membrane structures and fluctuation amplitudes on small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) profile is investigated based on harmonic motions of the surfactant bilayers with bending as well as thickness fluctuation motions. In this study we consider the case in which the wavelength of the bilayer membrane is shorter than the thickness of the membrane. We find that the thickness of the surfactant bilayer membrane, d(m), affects both q(dip) and q(peak) of I(q,0) profile, and that the fluctuation amplitude, a, of the membrane changes the peak of I(q,0). A simple formula is derived to estimate the thickness of the bilayer based on the q(dip) of the profile obtained from the simulation. The resulting estimates of the thickness of the bilayer with harmonic motion showed accuracy within 1%. Moreover, the bilayer thicknesses estimated from the proposed formula show an excellent agreement with the SANS and SAXS experimental results available in the literatures. We also propose a curve fit model, which describes the relationship between the fluctuation amplitude and the normalized q(peak) ratio. The present results show the feasibility of the simple formula to estimate the fluctuation amplitude based on the SANS and SAXS profiles.